Conventionally, machines driven by rotating a rotation shaft have been used, such as bicycles, automobiles, electric fans, belt conveyors, escalators and kneading machines. For example, a user drives a bicycle to run on the ground by turning a pedal which is a rotational driving source to generate propulsive power by transmitting a rotational driving force therefrom to the front and rear wheels through a chain. The bicycles as used in this manner include the so-called assist bicycle equipped with an electric motor as a rotational driving source and a battery to reduce the efforts required for turning a pedal when starting or climbing up a slope. The pedaling force exerted on this assist bicycle by human power is detected by a detection unit (torque sensor or the like), which outputs a signal indicative of the pedaling force to a control device which drives the electric motor in accordance with the pedaling force (for example, refer to the Patent Document 1).
Incidentally, in the case of the machines which are rotationally driven as described above, when the power source is stopped, the transmitted driving force is lost, conventionally, an electric power of no lower than a predetermined level has to be constantly supplied in order to successively operate the device.
In order to solve the problem as described above, the inventors developed the rotation assistance mechanism that can impart momentum to the rotating shaft to assist the rotation of the machine thereof and improve the usefulness (Patent Document 2). In the present invention, the distance between magnetic members which are paired with opposite poles facing each other is made to change in predetermined cycles by the rotation of the bicycle wheels, by providing a shield unit having a pair of magnetic surfaces the poles of which are the opposite as the respective opposing surfaces of the magnetic members, and a shield drive unit configured to advance the shield unit to between the pair of magnetic members in predetermined cycles when the distance between the pair of magnetic members is contracted, and retract the shield unit from between the pair of magnetic members in the predetermined cycles when the distance between the pair of magnetic members is expanded.
In accordance with the invention as described above, since the shield unit is provided with the pair of magnetic surfaces facing the pair of magnetic members respectively and having magnetic polarities which are opposite to those of the corresponding surfaces of the pair of magnetic members respectively, the shield drive unit shields the attracting magnetic force by advancing the shield unit to between the pair of magnetic members when the distance between the pair of magnetic members is contracted, and generates a repulsive magnetic force between the pair of magnetic members with same poles facing each other by retracting the shield unit from between the pair of magnetic members when the distance between the pair of magnetic members is expanded. As a result, for example, by providing a rotation assistance mechanism to the bicycle, it is possible to pedal without requiring effort for some time.